Method and apparatus for symmetrical DMT X-DSL communications

ABSTRACT

A method and apparatus for communicating multi-tone modulated upstream and downstream channels of communication data between a pair of communication devices utilizing a common set of tones for the upstream and downstream channels. The pair of communication devices each include a digital stage configured to assign mutually orthogonal code sequences for encoding and decoding the upstream and downstream channel respectively. The transmit path of the digital stage of each communication device is configured to generate redundancy in the associated communication data in either the time or frequency domain and to encode the redundant communication data with the mutually orthogonal code sequence prior to transmission thereby allowing the communication devices to share a common frequency spectrum of a communication medium for the upstream and downstream communication channels.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.09/757,018 filed Jan. 8, 2001 now abandoned entitled “Method andApparatus for Symmetrical DMT X-DSL Communications” which claims thebenefit of prior filed Provisional Applications No. 60/175,048 filed onJan. 7, 2000 entitled “FULL SPECTRUM X-DSL TRANSMISSION”. Each of theabove-cited applications is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates generally to X-DSL communications, and moreparticularly, X-DSL communications employing a DMT line code.

2. Description of the Related Art

North American Integrated Service Digital Network (ISDN) Standard,defined by the American National Standard Institute (ANSI), regulatesthe protocol of information transmissions over telephone lines. Inparticular, the ISDN standard regulates the rate at which informationcan be transmitted and in what format. ISDN allows full duplex digitaltransmission of two 64 kilo bit per second data channels. These datarates may easily be achieved over the trunk lines, which connect thetelephone companies' central offices. The problem lies in passing thesesignals across the subscriber line between the central office and thebusiness or residential user. These lines were originally constructed tohandle voice traffic in the narrow band between 300 Hz to 3000 Hz atbandwidths equivalent to several kilo baud.

Digital Subscriber Lines (DSL) technology and improvements thereonincluding: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. all ofwhich are broadly identified as X-DSL have been developed to increasethe effective bandwidth of existing subscriber line connections, withoutrequiring the installation of new fiber optic cable. An X-DSL modemoperates at frequencies higher than the voice band frequencies, thus anX-DSL modem may operate simultaneously with a voice band modem or atelephone conversation.

X-DSL modems are typically installed in pairs, with one of the modemsinstalled in a home and the other in the telephone companies centraloffice (CO) switching office servicing that home. This provides a directdedicated connection to the home from a line card at the central officeon which the modem is implemented through the subscriber line or localloop. Modems essentially have three hardware sections: (a) an analogfront end (AFE) to convert the analog signals on the subscriber lineinto digital signals and convert digital signals for transmission on thesubscriber line into analog signals, (b) digital signal processing (DSP)circuitry to convert the digital signals into an information bit streamand optionally provide error correction, echo cancellation, and lineequalization, and (c) a host interface between the information bitstream and its source/destination. Typically all of these components arelocated on a highly integrated single line card with a dedicatedconnection between one or more AFE's and a DSP.

Within each X-DSL protocol there are at least two possible line codes,or modulation protocols; i.e. discrete multi-tone (DMT) and carrierlessAM/PM (CAP). The first of these line codes, i.e. DMT, requires the DSPto implement both an inverse fast Fourier transform (IFFT) on upstreamdata received from the subscriber and a fast Fourier transform (FFT) onthe downstream data transmitted to the subscriber. Typically the DSP isavailable as a discrete semiconductor chip which implements thetransforms for a dedicated one of the X-DSL standards using softwareroutines running on an internal processor.

Each X-DSL installation represents a sizeable expense in hardware andservice labor to provision the central office. The expense may notalways be amortized over a sufficient period of time due the relentlessintroduction of new and faster X-DSL standards each of which pushes theperformance boundaries of the subscriber line in the direction ofincreasing bandwidth and signal integrity. As each new standardinvolves, line cards must typically be replaced to upgrade the service.

When X-DSL was first implemented the typical user profile was that of ahome or business user making intermittent access to the Internet over asubscriber line. Since the communication was extremely asymmetrical asto bandwidth requirement many of the popular X-DSL protocols providedsignificantly greater bandwidth from the central office to thesubscriber than in the opposite direction. User profiles are changing asmore and more users business and individual are becoming significantcontent providers or distributors as well as users.

What is needed are communication techniques which provide options forusers who require higher upstream bandwidth.

SUMMARY OF THE INVENTION

The current invention provides a method and apparatus for communicatingtwo or more channels of DMT modulated data within the same frequencyspectrum, thus providing symmetrical bandwidth for upstream anddownstream communication across a communication medium. The apparatusmay be used for dual channel or multi-channel communications. The methodmay be implemented on a physical modem or a logical modem with thelogical modem including a digital signal processor (DSP) coupled to ananalog front end (AFE). The communication medium may include: wired,wireless and optical. Orthogonality in either the time or frequencydomains is injected into the individual symbols associated with each DMTtone set or between successive tone sets using a unique code, e.g. Walshcode, assigned to each transmitted channel. The mutual orthogonality ofthese codes allows two or more channels to be supported in either anupstream or downstream direction using a DMT line code, in connectionwith any of the various X-DSL protocols including: G.Lite, ADSL, VDSL,SDSL, MDSL, RADSL, HDSL, etc.

In an embodiment of the invention a communication device for coupling toa communication medium and communicating at least two channels of datamodulated with DMT line code using a common set of tones for both atransmit path and a receive path is disclosed. The communication deviceincludes an analog stage and a digital stage. The analog stage convertson the transmit path digitized DMT symbols in the time domain to analogsignals. The analog stage also converts on the receive path analogsignals into the digitized DMT symbols in the time domain. The digitalstage generates a selected one of time domain redundancy and frequencydomain redundancy within the DMT line code for both the transmit pathand the receive path to obtain symmetrical bandwidth on the transmitpath and the receive path.

In an alternate embodiment of the invention a method for communicatingat least two channels of data between at least two modems is disclosed.Each modem includes a transmit path and a receive path and each of theat least two modems implement DMT modulation and demodulation on thetransmit path and the receive path respectively. The method forcommunicating comprises the acts of:

implementing a common set of sub carriers for communications of the atleast two channels of data between the modems and each sub carrierwithin the common set of sub carriers correlating with a respective tonewithin a common set of DMT tones;

generating one of time domain redundancy and frequency domain redundancyamong the DMT tones transmitted and received by each of said modems toobtain symmetrical bandwidth for communications between said at leasttwo modems across the common set of sub carriers.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more apparent to those skilled in the art from the followingdetailed description in conjunction with the appended drawings in which:

FIG. 1 depicts an overall system environment in which individualsubscribers are coupled across public service telephone network (PSTN)subscriber lines with one or more high speed networks.

FIG. 2 depicts a more detailed view of a representative one of thecentral offices shown in FIG. 1 including both digital subscriber lineaccess modules (DSLAMs) and PSTN voice band modules.

FIG. 3 is an expanded hardware view of one of the line cards in thecentral office shown in FIG. 2 which supports code division multipleaccess (CDMA) for discrete multi-tone “DMT” line codes.

FIG. 4 is an expanded hardware view of the digital signal processorportion (DSP) portion of the line card shown in FIG. 3.

FIG. 5 shows two logical modems each of which supports redundancy ineither the time or frequency domain for symmetric DMT line codes, andboth of which are coupled to one another across a subscriber line.

FIG. 6A is a graph of a prior art asymmetrical DMT line code showingboth the upstream and downstream frequency ranges with representativetones.

FIG. 6BC are graphs of the line code in which multiple access isaccomplished via redundancy in the frequency domain and the time domainrespectively, to enable symmetrical use of the frequency ranges shown inFIG. 6A.

FIG. 7AB show an embodiment of the transmit and receive logic associatedwith redundancy for DMT line codes implemented in the frequency and timedomains respectively.

FIG. 8 is a process flow diagram showing the processes associated withimplementing multiple access via redundancy for DMT line codes in eitherthe time or frequency domains.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The current invention provides a method and apparatus for communicatingtwo or more channels of DMT modulated data within the same frequencyspectrum, thus providing symmetrical bandwidth for upstream anddownstream communication across a communication medium. The apparatusmay be used for dual channel or multi-channel communications. The methodmay be implemented on a physical modem or a logical modem with thelogical modem including a digital signal processor (DSP) coupled to ananalog front end (AFE). The communication medium may include: wired,wireless and optical. Orthogonality in either the time or frequencydomains is injected into the individual symbols associated with each DMTtone set or between successive tone sets using a unique code, e.g. Walshcode, assigned to each transmitted channel. The mutual orthogonality ofthese codes allows two or more channels to be supported in either anupstream or downstream direction using a DMT line code, in connectionwith any of the various X-DSL protocols including: G.Lite, ADSL, VDSL,SDSL, MDSL, RADSL, HDSL, etc. The present invention provides a signalprocessing architecture that supports scalability of central office (CO)or, Digital Loop Carrier (DLC) or, Optical Network Units (ONU)resources, and allows a significantly more flexible hardware response tothe evolving X-DSL standards without over committing of hardwareresources. As standards evolve hardware may be reconfigured to supportthe new standards.

FIG. 1 depicts an overall system environment in which individualsubscribers are coupled across public service telephone network (PSTN)subscriber lines with one or more high speed networks. Telco COs 100,102, 106 and remote access terminal 104 are shown coupling varioussubscribers to one another and to a high speed network 140. The highspeed network 140 provides fiber optic links between the central officeand remote access terminal. CO's 100–102 are coupled to one another viafiber optic link 142. CO 102 couples to remote access terminal 104 viafiber optic link 146. CO also couples to subscriber site 122 via fiberoptic link 144. CO 102 and CO 106 couple to one another via a wirelesslink provided by corresponding wireless transceivers 130 and 132respectively. The “last mile” connecting each subscriber, (exceptsubscriber 122) is provided by twisted copper PSTN telephone lines. Onthese subscriber lines voice band and data communication are provided.The data communication is shown as various X-DSL protocols includingG.Lite, ADSL VDSL, and HDSL2. CO 100 is coupled via G.Lite and ADSLmodulated subscriber line binder 160 with subscribers 110 and 112. CO100 is also coupled via G.Lite and ADSL modulated subscriber line binder162 with subscriber 114. CO 106 is also coupled via a subscriber line tosubscriber 134. Remote access terminal is coupled via subscriber linebinder 164 with subscribers 120. In each case the corresponding CO mayadvantageously be provided with distributed AFE and DSP resources forhandling multiple protocols from multiple locations with the addedbenefit of load balancing, and statistical multiplexing. The apparatusand method of the current invention is suitable for handlingcommunications on any of these subscriber lines. Communications are alsoprovided between DSP resources at one site, e.g. CO 100 and AFEresources at a separate site, e.g. CO 102. This later capability allowsdistributed processing whereby all DSP resources can be placed in alogical server environment hence supporting a client serverarchitecture.

FIG. 2 depicts a more detailed view of a representative one of thecentral offices shown in FIG. 1 including both digital subscriber lineaccess modules (DSLAMs) and PSTN voice band modules. The CO 100 includessubscriber line connections to subscribers 110–114. Subscriber line 262couples subscriber 110 with the CO. Each of these connections terminatesin the frame room 200 of the CO. From this room connections are made foreach subscriber line via splitters and hybrids to both a DSLAM 202 andto the voice band racks 204. The splitter shunts voice bandcommunications to dedicated line cards, e.g. line card 242 or to a voiceband modem pool (not shown). The splitter shunts higher frequency X-DSLcommunications on the subscriber line to a selected line card 210 withinDSLAM 202. The line cards of the current invention are universal,meaning they can handle any current or evolving standard of X-DSL andmay be upgraded on the fly to handle new standards.

Voice band call set up is controlled by a Telco switch matrix 240 suchas SS7. This makes point-to-point connections to other subscribers forvoice band communications on the Public Switched Telephone Network(PSTN) 260. The X-DSL communications may be processed by a universalline card such as line card 212. That line card includes a plurality ofAFE's e.g. 212–214 each capable of supporting a plurality of subscriberlines. The AFEs are coupled via a proprietary packet based bus 216 to aDSP 218 which is also capable of multi-protocol support for allsubscriber lines to which the AFE's are coupled. The line card itself iscoupled to a back-plane bus 220 which may in an embodiment of theinvention be capable of offloading and transporting low latency X-DSLtraffic between other DSPs for load balancing. Communications betweenAFE's and DSP(s) are packet based which allows a distributedarchitecture such as will be set forth in the following FIG. 3 to beimplemented. Each of the DSLAM line cards operates under the control ofa DSLAM controller 200 which handles global provisioning, e.g.allocation of subscriber lines to AFE and DSP resources. Once an X-DSLconnection is established between the subscriber and a selected one ofthe DSLAM submodules, e.g. AFE and DSP the subscriber will be able toaccess any network to which the DSLAM is connected. In the example shownthe DSLAM couples via server 230 with Internet 140.

FIG. 3 is a chip level view of an embodiment of the inventionimplemented within a logical modem formed on line card 210 by the DSP218 and AFE 214. The AFE's chips 212–214 connect with a DSP chip 218across bus 216. They all may be mounted on the line card 210 shown inFIG. 2. Packets of raw data are shown being transported between the DSPand AFEs as well as within each DSP and AFE. Packet processing betweenthe DSP and AFE chips involves transfer of bus packets 300. Packetprocessing within a DSP may involve device packets 306 (See FIG. 5).Packet processing within an AFE may involve raw data packets 302. Thesewill be discussed in the following text.

These modules, AFE and DSP, may be found on a single universal linecard, such as line card 210 in FIG. 2. They may alternately be displacedfrom one another on separate line cards linked by a DSP bus. In stillanother embodiment they may be found displaced from one another acrossan ATM network. There may be multiple DSP chipsets on a line card. In anembodiment of the invention the DSP and AFE chipsets may includestructures set forth in the figure for handling of multiple line codesand multiple channels.

The DSP chip 218 includes an upstream receive path and a downstreamtransmit path with both discrete and shared components. Data for each ofthe channels is passed along either path in discrete packets the headersof which identify the corresponding channel and may additionally containchannel specific control instructions for various of the shared anddiscrete components along either the transmit or receive path.

On the upstream path, upstream packets from various of the subscribersare received by the DSP medium access control (MAC) 314 which handlespacket transfers to and from the DSP bus. These packets contain digitaldata corresponding with DMT symbols expressed in the time domain. Thereis redundancy in one of the frequency or the time domain for theupstream DMT symbols. Where redundancy in the frequency domain isimplemented a single set of DMT symbols contains redundancy of order Rfor the symbol set. Where redundancy is expressed in the time domainsuccessive sets of DMT symbols contain redundancy of order R. Thisredundancy implemented with orthogonal coding between channels carriedon the subscriber line allows multiple channels to be carried in eitherthe upstream or downstream direction on the subscriber line 262. The MACcouples with a packet assembler/disassembler (PAD) 316. For upstreampackets, the PAD handles removal of the DSP bus packet header andinsertion of the device header and control header which is part of thedevice packet 306. The content of these headers is generated by the coreprocessor 334 using information downloaded from the DSLAM controller 200(See FIG. 2) as well as statistics such as gain tables gathered by thede-framer 332, or embedded operations channel communications from thesubscriber side. These channel specific and control parameters 326 arestored in memory 328 which is coupled to the core processor. The PAD 316embeds the required commands generated by the core processor in theheader or control portions of the device packet header of the upstreamdata packets. The upstream packets may collectively include data frommultiple channels each implementing various of the X-DSL protocols bothwith and without time and/or frequency domain redundancy depending onthe subscriber line from which they originated.

The header of each device packet identifies the channel correspondingwith the data contained therein. Additionally, a control portion of thepacket may include specific control instructions for any of the discreteor shared components which make up the upstream or downstream processingpaths. In the embodiment shown, the Fourier transform engine (FTE) 322is a component which is shared between the upstream and downstreampaths. Thus, on the upstream path each upstream packet is delivered tothe FTE for demodulation. The FTE handles the mapping of data and theprocessing of the packets as it flows through the FTE. The informationin the header of the packet is used to maintain the channel identity ofthe data as it is demodulated, to setup the FTE at the appropriateparameters for that channel, e.g. sample size, and to provide channelspecific instructions for the demodulation of the data. The demodulateddata is passed as a packet to the next component in the upstream path,e.g. the Walsh decoder. Where redundancy in one of the time andfrequency domain exists in an incoming channel the Walsh decoder 338removes that redundancy by combining the code for the transmittedchannel with the received data to remove the redundancy. The variousWalsh codes for each received channel which implements redundancy may beprovided during session setup along with the Walsh codes for thetransmission of each redundant channel. Next the digitized DMT symbolsare decoded, reordered and deframed within the remainder of the deframerdecoder 332. Each component in the receive path reads the next devicepacket and processes the data in it in accordance with the instructionsor parameters in its header. The demodulated, decoded and de-framed datais passed to the asynchronous transfer mode (ATM) PAD 340. In the ATMPAD the device packet header is removed and the demodulated datacontained therein is wrapped with an ATM header. The packet is thenpassed to the ATM MAC 344 for transmission of the ATM packet on the ATMnetwork 140 (See FIGS. 1–2).

On the downstream path, downstream packets containing digital datadestined for various subscribers is received by the ATM MAC 344 whichhandles transfers to and from the ATM network 140. The ATM MAC passeseach received packet to the ATM PAD 340 where the ATM header is removedand the downstream device packet 306 is assembled. Using header contentgenerated by the core processor 334 the PAD assembles data from the ATMnetwork into channel specific packets each with their own header, dataand control portions. The downstream packets are then passed to theFramer, tone orderer, and Reed Solomon encoder 336 where they areprocessed in a manner consistent with the control and header informationcontained therein.

Where redundancy in one of the time and frequency domain exists in thedownstream channel the Walsh encoder injects the required redundancy bycombining the code for the transmitted channel with the data from theframe encode block 336. The redundant data in the form of redundanttones within a DMT tone set or redundant sets of DMT tones are thenpassed to the input of the FTE. The FTE governs the multiplexing ofthese downstream packets which will be modulated by the FTE with theupstream packets which will be demodulated therein. Each downstreampacket with the modulated data contained therein is then passed to theDSP PAD 316. In the DSP PAD the device packet header and controlportions are removed, and a DSP bus header 304 is added. This headeridentifies the specific channel and may additionally identify thesending DSP, the target AFE, the packet length and such otherinformation as may be needed to control the receipt and processing ofthe packet by the appropriate AFE. The packet is then passed to the DSPMAC for placement on the DSP bus 216 for transmission to the appropriateAFE.

FIG. 3 also shows a more detailed view of the processing of upstream anddownstream packets within the AFE. In the embodiment of the inventionshown, device packets are not utilized in the AFE. Instead, channel andprotocol specific processing of each packet is implemented using controlinformation for each channel stored in memory at session setup.Downstream packets from the DSP are pulled off the bus 216 by thecorresponding AFE MAC on the basis of information contained in theheader portion of that packet. The packet is passed to AFE PAD 346 whichremoves the header 304 and sends it to the core processor 372. The coreprocessor matches the information in the header with channel controlparameters 362 contained in memory 360. These control parameters mayhave been downloaded to the AFE at session setup. The raw data 302portion of the downstream packet is passed to FIFO buffer 352 under themanagement of controller 350. Each channel has a memory mapped locationin that buffer. The interpolator and filter 358 reads a fixed amount ofdata from each channel location in the FIFO buffer. The amount of dataread varies for each channel depending on the bandwidth of the channel.The amount of data read during any given time interval is governed bythe channel control parameters 362, discussed above. The interpolatorupsamples the data and low pass filters it to reduce the noiseintroduced by the DSP. Implementing interpolation in the AFE as opposedto the DSP has the advantage of lowering the bandwidth requirements ofthe DSP bus 216. From the interpolator data is passed to the FIFO buffer368 under the control of controller 366. The downstream packets 370 mayincrease in size as a result of the interpolation. The next module inthe transmit pipeline is a DAC 378 which processes each channel inaccordance with commands received from the core processor 372 using thecontrol parameters downloaded to the control table 362 during channelsetup. The analog output of the DAC is passed via analog mux 384 to acorresponding one of sample and hold devices 386. Each sample and holdis associated with a corresponding subscriber line. The sampled data isfiltered in analog filters 390 and amplified by line amplifiers 394. Theparameters for each of these devices, i.e. filter coefficients,amplifier gain etc. are controlled by the core processor using the abovediscussed control parameters 362. For example, where successivedownstream packets carry downstream channels each of which implementsdifferent protocols, e.g. G.Lite, ADSL, and VDSL the sample rate of theanalog mux 384, the filter parameters for the corresponding filter 390and the gain of the corresponding analog amplifiers 394 will vary foreach packet. This “on the fly” configurability allows a singledownstream pipeline to be used for multiple concurrent protocols.

On the upstream path many of the same considerations apply. Individualsubscriber lines couple to individual line amplifiers 396 throughsplitter and hybrids (not shown). Each channel is passed through analogfilters (not Shown), sample and hold modules 388 and dedicated ADCmodules 380–382. As discussed above in connection with thedownstream/transmit path, each of these components is configured on thefly for each new packet depending on the protocol associated with it.Each upstream packet is placed in a memory mapped location of FIFOmemory 374 under the control of controller 376. From the controllerfixed amounts of data for each channel, varying depending on thebandwidth of the channel, are processed by the decimator and filtermodule 364. The amount of data processed for each channel is determinedin accordance with the parameters 362 stored in memory 360. Thoseparameters may be written to that table during the setup phase for eachchannel.

From the decimator and filter the raw data 302 is passed to FIFO buffer354 which is controlled by controller 356. Scheduled amounts of thisdata are moved to PAD 348 during each bus interval. The PAD wraps theraw data in a DSP header with channel ID and other information whichallows the receiving DSP to properly process it. The upstream packet isplaced on the bus by the AFE MAC 346. A number of protocols may beimplemented on the bus 216. In an embodiment of the invention the DSPoperates as a bus master governing the pace of upstream and downstreampacket transfer and the AFE utilization of the bus.

FIG. 4 is an expanded hardware view of the digital signal processorportion (DSP) of the line card shown in FIG. 3. Subcomponents of each ofthe DSP Pad 316, the FTE 322, the Deframer-decoder 332, theframer-encoder 336 and the ATM PAD 340 are shown.

On the upstream packet path, the DSP PAD includes a first-in-first-out(FIFO) buffer 400 where upstream packets from the AFEs are stored and acyclic prefix remover 404. After removal of the cyclic prefix eachpacket is then passed to the DFT mapper 424. The DFT mapper is coupledto the input memory portion of the FTE via a multiplexer 420. The mapperhandles writing of each sample set from a packet into the input memoryin the appropriate order. The mapper may also handle such additionalfunctions as time domain equalization (TEQ) filtering which is a digitalprocess designed to normalize the impact of differences in channelresponse. The filter may be implemented as an FIR filter. The inputmemory comprises two portions 416 and 418. Multiplexer 420 providesaccess to these memories. While one sample set, e.g. time or frequencydomain data, is being written from the upstream or downstream data pathsinto one of the memories the contents of the other of the memories arewritten into the row and column component 412 of the FTE 322. Once theDFT is completed by the row and column component the frequency domaincoefficients generated thereby are stored in either of portions 410–412of the output memory of the FTE. These coefficients correspond with eachof the DMT sub carriers or tones. A demultiplexer 408 handles thecoupling of the output memory to either the next component of theupstream path, i.e. the deframer-decoder 332 or of the downstream path.Next on the upstream path, the device packet with header and dataportions and optional control portion is passed to the remainingcomponents of the upstream path. These include the gain scalar andoptional forward error correction (FEQ) 426, the Walsh decoder 338, thetone decoder 428, the tone re-orderer 430 and the deframer 434.

A multiplexer 432 couples the deframer input to either the tonereorderer 430 or to the output memory of the FTE. Each of thesecomponents is individually configurable on a per channel basis usingtables stored locally in registers within each component, or withinmemory 328. The access to these tables/registers is synchronized by thelogic in each of the components which responds to header or controlinformation in each upstream packet to alter tone ordering/re-ordering,gain scaling constants per-tone per-channel, and FEQ constants per-toneper-channel. The processor 334 may initialize all the registers. Fromthe deframer packets are passed to the FIFO buffer 450 which is part ofATM PAD 340.

The core processor 334 has DMA access to the FIFO buffer 450 from whichit gathers statistical information on each channel including gaintables, or gain table change requests from the subscriber as well asinstructions in the embedded operations portion of the channel. Thosetables 326 are stored by the core processor in memory 328. When a changein gain table for a particular channel is called for the core processorsends instructions regarding the change in the header of the devicepacket for that channel via PAD 316. The core processor 334 then writesthe new gain table to a memory, e.g. memory 326, which can be accessedby the appropriate component, e.g. FTE 322 or Gain Scalar 426. As thecorresponding device packet is received by the relevant component thatcomponent, e.g. the gain scalar applies the updated parameters toappropriately scale the data portion of the packet and all subsequentpackets for that channel. This technique of in band signaling withpacket headers allows independent scheduling of actions on a channel bychannel basis in a manner which does not require the direct control ofthe core processor. Instead each module in the transmit path can executeindependently of the other at the appropriate time whatever actions arerequired of it as dictated by the information in the device header whichit reads and executes.

On the downstream path a FIFO buffer 452 within the ATM PAD 340 holdsincoming packets. These are passed to the components in the Framer andEncoder module 336 for processing. The components of that module includethe framer 440, tone orderer 442, tone encoder 444, Walsh encoder 342and gain scalar 446. They are coupled via a multiplexer 448 to the IDFTmapper 422. As was the case with the deframer, the framer will useprotocol specific information associated with each of these channels tolook for different frame and super frame boundaries. The tone orderersupports varying number of tones, bytes per tone and gain per tone foreach of the X-DSL protocols. For example the number of tones for G.Liteis 128, for ADSL is 256 and for VDSL 2048. The number of bits to beextracted per tone is read from the tone-ordering table or register atthe initiation of processing of each packet. For example as successivepackets from channels implementing G.Lite, ADSL and VDSL pass throughthe DMT Tx engine the number of tones will vary from 128 for G.lite, to256 for ADSL, to 2048 for VDSL. In the encoder 444 constellation mappingis performed based on the bit pattern of each packet. The output is atwo dimensional signal constellation in the complex domain.

Next in the IDFT mapper each device packet is correlated with a channeland protocol and mapped into input memory via a connection provided bymultiplexer 420. The mapping is in a row and column order. Next in theFTE, the complex digital symbols DMT symbols are modulated into carriersor tones in the row and column transform component 414 and placed ineither portion 410 or 412 of output memory having been transformed fromthe frequency to the time domain. The dimensions of the row and columntransforms vary on a channel specific basis. Next a packet with thememory contents, i.e. the digitized DMT symbols transformed to the timedomain is passed as a packet via demultiplexer 408 to the DSP FIFObuffer 406. This is part of DSP PAD 316. Individual packets are movedfrom this buffer to the cyclic prefix component 402 for the addition ofthe appropriate prefix/suffix. The cyclic prefix component is responsiveto the device packet header which identifies the channel for which datais being processed. This can be correlated with the requiredprefix/suffix extensions for the protocol associated with the channel onthe basis of parameters 326 stored in main memory 328 or withindedicated registers in the component. For example the cyclic extensionfor G.Lite is 16, for ADSL 32, and for VDSL 320.

This device architecture allows the DSP transmit and receive paths to befabricated as independent modules or submodules which respond to packetheader and or control information for processing of successive packetswith different X-DSL protocols, e.g. a packet with ADSL sample datafollowed by a packet with VDSL sampled data A mixture of differentcontrol techniques are used to control the behavior of the individualcomponents of the DSP. The packet header may simply identify thechannel. The component receiving the packet may then reference internalregisters or downloaded tables such as table 326 to correlate thechannel with a protocol and the protocol with the correspondingparameters with which the data portion of the packet is to be processed.Alternately the device packet may contain specific control informationsuch as that associated with shutting down a channel, idling a channel,or shutting down the DSP.

FIG. 5 shows two logical modems each of which supports redundancy ineither the time or frequency domain for symmetric DMT line codes, andboth of which are coupled to one another across a subscriber line 262.The components and their functioning are identical to those discussedabove with the exception that discrete IDFT and DFT modules 500A–B and502A–B respectively are shown to simplify the following explanation ofmultiple access via frequency or time domain redundancy.

FIG. 6A is a graph of a prior art asymmetrical DMT line code showingboth the upstream and downstream frequency ranges with representativetones. An upstream set of tones 600 defined in an upstream frequencyrange from the subscriber to the CO is defined between a lower frequencyf₁ and an upper frequency f₂. An individual upstream tone 604 is shown.This is transmitted with its own sub-carrier and individual quadratureamplitude modulated (“QAM”) symbol is shown. A downstream set of tones602 defined in an downstream frequency range from the CO to thesubscriber between a lower frequency f₃ and an upper f₄ is shown.Several individual downstream tones 606, 608, 612, 610 are shown. Eachcarries varying numbers of bits allocated by the tone orderer of therespective modem. These also are transmitted with individual sub-carrierand individual quadrature amplitude modulated (“QAM”) symbol defined bythe tone encoder. The upstream and downstream tones are asymmetrical.For G.Lite for example there are 32 upstream tones 96 downstream tonesfor a total of 128 tones. For ADSL there are 32 upstream tones and 224downstream tones for a total of 256 tones. VDSL admits two additionalfrequency ranges for upstream and downstream communications.

FIGS. 6BC and 6C are graphs of the line code in which multiple access isaccomplished via redundancy in the frequency domain and the time domainrespectively, to enable symmetrical use of the frequency ranges shown inFIG. 6A. In FIG. 6B the combined RN tones of both the upstream anddownstream frequency ranges 600 and 602 are used for both upstream anddownstream communications. Each individual tone 604, 606–612 for examplecontains redundancy of order R within the frequency domain for a totalof RN tones. The tones are shown as stacked, consisting of a singlesymbol which corresponds with both the upstream and downstream componentof the 2 channels communicated on the shared communication medium. Forthe case of two channels sharing the communication medium each upstreamand downstream tone is redundantly expressed in at least two tones.Which tones contain redundant data is information which the Walshdecoder shown in the previous FIG. 5 requires in order to removeredundancy from digital DMT symbols in the frequency domain which itreceives from the DFT 502A–B. The redundancy is implemented with theappropriate sign convention to maintain orthogonality, between the twoor more channels which share the combined tone sets 600 and 602.

FIG. 6C shows redundancy implemented in the time domain. Here each toneset includes a number of tones equal to I/R times the number of tonesshown in FIG. 6B where R is the order of the redundancy. In the twochannel examples shown in the following text the order of redundancy is2 therefore there are ½ the tones shown in FIG. 6B. Implementation ofmultiple access via redundancy in the time domain therefore requires inthis case ½ the number of tones within each combined tone set 600–602and the transmission of two, in this case, tone sets at times t₁ and t₂.Here also the number of tones required for redundancy is RN. These twotone sets contain the same cumulative number of tones RN as wasassociated with frequency domain bases multiple access method shown inFIG. 6B.

FIGS. 7 and 7B show an embodiment of the transmit and receive logicassociated with redundancy for DMT line codes implemented in thefrequency and time domains respectively.

Two logical modems are shown identical to the modems shown in FIG. 5.The first logical modem includes DSP 218A together with AFE 214A. Thesecond logical modem includes DSP 218B together with AFE 218B. Themodems are coupled to one another via a common communication medium,e.g. subscriber line 262. Exploded views of a hardware implementation ofthe multiple access decoders 338A–B and encoders 342A–B are shown withintheir corresponding modem. In the embodiment shown the encoders take anincoming set of DMT symbols for each tone. Each symbol may be expressedas a complex number x+ij.

In FIG. 7A multiple access via frequency domain redundancy isimplemented using the DMT line code as discussed above in FIG. 6B. Inthis embodiment the incoming data stream is framed in framer 440A.Individual portions of the framed data are allocated to a correspondingtone bin by the tone orderer 442A and passed to the DMT tone encoder444A where they are mapped to a DMT symbol and expressed as a complexnumber. In the example shown the number of DMT tones processed in eachtone set by the tone ordered 442A is two shown within the frameboundaries 700A–702A. The two DMT tones are labeled A₁, A₀ with A₀ thefirst of the DMT symbols to be transmitted. Redundancy in the frequencydomain is injected to this set of tones by appropriate switching of theWalsh encoder 342A in accordance with the selected Walsh code drivingthe demultiplexer 710A and multiplexer 712A which switch each incomingsymbol of the symbol set from a non-inverted path 718A to an invertedpath 716A which includes inverter 714A. This implements redundancy R oforder 2 in the frequency domain for a total of RN tones, in this case 4within a single tone set. The encoder generates a tone set with twicethe number of tones per tone set as input as shown within frameboundaries 704A–706A. In the example shown these are: −A₁, A₁, −A₀, A₀,for a total of 4 DMT symbols. The sign convention corresponds with aWalsh code of +/− 1s assigned to DSP 218A for transmission of data. TheWalsh code for the DSP 218B is orthogonal to that assigned to DSP 218A.The sequence of DMT tones transmitted by that modem is: B₁, B₁, B₀, B₀.The decoders 338A–B include corresponding buffers 722A–B and 720A–B,along with summers 724-A1, subtractor 724-A2 and corresponding dividers726A–B. The decoder accepts 4 DMT symbols each of which includescontribution from both the transmit path of the opposite modem as wellas from Self NEXT from the modems own transmitter with which the DMTtone set is shared. In the example shown the received tones are: A₀₊B₀,A₀₊B₀, A₁₊B₁, ⁻A₁₊B₁. The decoder decodes the incoming sets of tonesredundant in the frequency domain. The redundancy and coding isestablished during session set up for each channel. The redundancy isremoved and the appropriate received DMT symbol sets B₀, B₁ are passedto the tone decoder 428A for decoding. The tone reorderer 430A performsreordering of the tones and the deframer 434A deframes the DMT symbols.On the receive path of the opposite modem the decoder 338B decodes withan orthogonal Walsh code. In alternate embodiments of the inventionredundancy in the frequency domain may be implemented at the toneorderer. In alternate embodiments of the invention code sequences otherthan Walsh coding may be implemented to introduce orthogonal redundancyinto the channels which share a common set of DMT tones.

In FIG. 7B multiple access via time domain redundancy is implementedusing the DMT line code as discussed above in FIG. 6C. In thisembodiment the incoming data stream is framed in framer 440A. Individualportions of the framed data are allocated to a corresponding tone bin bythe tone orderer 442A and passed to the DMT tone encoder 444A where theyare mapped to a DMT symbol and expressed as a complex number. In theexample shown the number of DMT tones processed in each tone set by thetone ordered 442A is two shown within the frame boundaries 700A–702A.The two DMT tones are labeled A₁, A₀ with A₀ the first of the DMTsymbols to be transmitted. Redundancy in the time domain is injected tothis set of tones by appropriate switching in accordance with theselected Walsh code of the Walsh encoder 342A. An additional multiplexer720A and input buffers 720A–722A have been added to the encoder enablingit to implement redundancy R of order 2 in the time domain. The encoder342A generates two tone sets with the same number of tones as the inputtone set for a total of RN tones, in this case 4. In the example shownthese are: −A₁, A₀ in a second tone set with boundaries 732A–734A andA₁, A₀, in a first tone set with boundaries 730A–732A. The signconvention corresponds with a Walsh code of +/− 1s assigned to DSP 218Afor transmission of data. The Walsh code for the DSP 218B is orthogonalto that assigned to DSP 218A. The sequence of DMT tones transmitted bythat modem is: B₁, B₁, B₀, B₀. The decoders 338A–B include correspondingbuffers 722A–B and 720A–B, along with summers 724-A1, subtractor 724-A2and corresponding dividers 726A–B. The decoder accepts 4 DMT symbolseach of which includes contribution from both the transmit path of theopposite modem as well as from Self NEXT from the modems own transmitterwith which the DMT tone set is shared. In the example shown the receivedtones are from first to last: A₀₊B₀, A₀₊B₀, A₁₊B₁, ⁻A₁₊B₁. The decoderdecodes the incoming sets of tones redundant in the time domain. Theredundancy and coding is established during session set up for eachchannel. The redundancy is removed and the appropriate received DMTsymbol sets B₀, B₁ are passed to the tone decoder 428A for decoding. Thetone reorderer 430A performs reordering of the tones and the deframer434A deframes the DMT symbols. On the receive path of the opposite modemthe decoder 338B decodes with an orthogonal Walsh code. In alternateembodiments of the invention redundancy in the time domain may beimplemented at the output of the IDFT instead of the input as discussedabove. In alternate embodiments of the invention code sequences otherthan Walsh coding may be implemented to introduce orthogonal redundancyinto the channels which share a common set of DMT tones.

FIG. 8 is a process flow diagram showing the processes associated withimplementing multiple access via redundancy for DMT line codes in eitherthe time or frequency domains. Processing begins at start block 800 andcontrol passes to decision process a 802. In decision process 802 adetermination is made as to whether symmetrical multiple access of morethan two channels within a single tone set is enabled. If not controlpasses to block 804 which marks the commencement of normal asymmetricfunctioning of all transmit and receive path portions of the modemsdiscussed above. If multiple access is enabled for at least two channelsto a single tone set then control passes to decision process 806. Inprocess 806 a determination is made is to whether redundancy in the timeor frequency domain will be implemented.

Frequency Domain:

For the case where redundancy in the frequency domain is implemented,control passes to process 810. In process 810 existing upstream anddownstream tones sets are combined into a single set of tones andvarious components of the transmit and receive path shown in FIGS. 3–5are configured accordingly. Control passes to process 812 in which theappropriate code words are assigned to the multiple access encoders anddecoders of each modem. Control then passes to process 814 for thecommencement of data transmission.

Data transmission begins with process 816 with the acceptance of thenext block of data for transmission. Control then passes to process 818in which redundancy in the frequency domain is injected into the tonesassociated with the block of data to be transmitted as shown in FIG. 6B.Next in process 820 one symbol set RN tones is transmitted. Control thenpasses to receive block 822.

Reception begins with process 822 from which control is passed toprocess 824 for reception of the next DMT symbol set of RN tones. R isthe order of redundancy in the frequency domain. Control next passes toprocess 826 in which the multiple access decoder correlates the codeword for the received channel with the digitized DM T symbol setreceived from the AFE and converted to the frequency domain by the DFTto remove redundancy in the frequency domain. Control then passes toprocess 828 in which tone decoding, reordering and de-framing areimplemented. Then in process 870 the next set of tones for transmissionor reception is processed.

Time Domain:

Multiple access via redundancy of the DMT line code in the time domaincommences after a determination to that effect in decision process 806.Control passes to process 840. In process 840 existing upstream anddownstream tones sets are combined into a single set of tones andvarious components of the transmit and receive path shown in FIGS. 3–5are configured accordingly. Control passes to process 842 in which theappropriate code words are assigned to the multiple access encoders anddecoders of each modem. Control then passes to process 844 for thecommencement of data transmission.

Data transmission begins with process 846 with the acceptance of thenext block of data for transmission. Control then passes to process 848in which redundancy in the time domain is injected into the tonesassociated with the block of data to be transmitted as shown in FIG. 6C.Next in process 850 R symbol sets with a total of RN tones aretransmitted. Control then passes to receive block 852.

Reception begins with process 852 from which control is passed toprocess 854 for reception of the next DMT symbol sets of RN tones. R isthe order of redundancy in the time domain. Control next passes toprocess 856 in which the multiple access decoder correlates the codeword for the received channel with the digitized DMT symbol setsreceived from the AFE and converted to the frequency domain by the DFTto remove redundancy in the time domain. Control then passes to process858 in which tone decoding, reordering and de-framing are implemented.Then in process 870 the next set of tones for transmission or receptionis processed.

In alternate embodiments of the invention the communications betweenvarious components on the transmit path and the receive path of eachlogical or physical modem are implemented with packet based transfers ofdata and symbols the invention may be applied with equal utility to aphysical or logical modem with any other form of modular or non-modularcommunication including point-to-point communication.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

1. A multi-tone X-DSL communication device with a plurality of sharedand discrete components forming a transmit path and a receive pathconfigured to couple to a communication medium to communicate multi-tonemodulated communication channels, and the multi-tone X-DSL communicationdevice comprising: a Walsh encoder on the transmit path and the Walshencoder configured to encode with a first Walsh codeword successivesymbols of a transmitted communication channel signal; an inverseFourier transform component on the transmit path coupled to the Walshencoder to transform encoded symbols of the transmitted communicationchannel signal from the frequency domain to the time domain fortransmission over the communication medium to an opposing communicationdevice on a common set of tones; a Fourier transform component on thereceive path, and the Fourier transform component configured to receivefrom the opposing communication device on the common set of tones acommunication channel signal encoded by the opposing communicationdevice with a second Walsh codeword orthogonal to the first Walshcodeword and to transform the received communication channel signal fromthe time domain to the frequency domain; and a Walsh decoder on thereceive path, and the Walsh decoder coupled to the Fourier transformcomponent and configured to decode a with the second Walsh codeword thereceived communication channel signal transformed by the Fouriertransform component, whereby the common set of tones of the transmittedand received communication channel signals span a shared frequency rangeon the communication medium.
 2. The multi-tone X-DSL communicationdevice of claim 1, further comprising: the Walsh encoder furtherconfigured to encode successive pairs of symbols with the first Walshcodeword thereby generating data redundancy on the transmit path in thetime domain between each successive pair of encoded symbols of thetransmitted communication channel signal; and the Walsh decoder furtherconfigured to decode successive pairs of encoded symbols of the receivedcommunication channel signal with the second Walsh codeword a therebyremoving data redundancy in the time domain from the receivedcommunication channel signal.
 3. The multi-tone X-DSL communicationdevice of claim 1, further comprising: the Walsh encoder furtherconfigured to encode each symbol with the first Walsh codeword therebygenerating data redundancy on the transmit path in the frequency domain,between tones within each encoded symbol; and the Walsh decoder furtherconfigured to decode each encoded symbol of the received communicationchannel signal thereby removing a data redundancy in the frequencydomain from the received communication channel signal.
 4. A method forcommunicating multi-tone modulated upstream and downstream communicationchannels between a first and second multi-tone X-DSL communicationdevices coupled to one another via a communication medium, and themethod for communicating comprising the steps of: establishing a firstWalsh codeword and a second Walsh codeword orthogonal to one another forencoding upstream and downstream communication channels signals betweenthe first and second multi-tone X-DSL communication devices; encodingtransmissions of the upstream communication channel signal with thefirst Walsh codeword and encoding transmissions of the downstreamcommunication channel signal with the second Walsh codeword establishedin the establishing step; transforming the upstream and downstreamcommunication channel signals between a frequency domain and a timedomain using a common set of tones spanning a shared frequency range onthe communication medium; and decoding reception of the upstreamcommunication channel signal with the first Walsh codeword and decodingreception of the downstream communication channel signal with the secondWalsh codeword.
 5. The method of claim 4, wherein the encoding anddecoding steps further comprises: encoding successive pairs of symbolstransmitted on the upstream and downstream communication channelssignals, thereby generating data redundancy in the time domain betweeneach successive pair of encoded symbols; and decoding successive pairsof encoded symbols received on the upstream and downstream communicationchannel signals, thereby removing the data redundancy in the timedomain.
 6. The method of claim 4, wherein the encoding and decodingsteps further comprises: encoding duplicating tones within each tone seteach symbol transmitted of on the upstream and downstream communicationchannels signals on the transmit paths of the first modem and secondmodem respectively thereby generating data redundancy in the frequencydomain between tones in each encoded symbols; and decoding each encodedsymbol received on the upstream and downstream communication channelsignals, thereby removing the data redundancy in the frequency domain.